Stone milling has been used for centuries to grind grains into flour. In recent years, there has been a resurgence of interest in this traditional method of milling due to its many benefits. Unlike modern industrial milling, stone milling preserves the nutrient-rich germ and bran of the grain, resulting in flour that is more nutritious and flavorful. In this blog, we will explore two categories of stones namely emery and granite, used in modern day stone milling across the globe and the benefits it offers in producing healthier flour.
Milling wheat with granite and emery stones can lead to variations in flour characteristics due to differences in stone composition and milling process. Granite stones are harder and denser than emery stones, resulting in a finer grind and a more uniform particle size distribution. They produce less heat during the milling process, which helps preserve the flour's nutrients and flavors.
In contrast, emery stones (a rock rich in corundum) are softer and more porous, leading to a coarser grind and a more uneven particle size distribution. The higher RPM emery mills produce more heat during the milling process, which can affect the flour's nutrient and flavor profile.
The choice of stone and milling process can affect the flavor, texture, and nutritional value of the resulting flour, with some bakers preferring artisanal stone-milled flour and others preferring industrially roller-milled flour.
Diving deeper into the granites:
Barre Grey granite is a type of granite from Barre, Vermont, USA, known for its durability, consistency, and fine-grained texture.
It is not possible to identify a geochemically equivalent of Barre Grey granite in Europe and India as the geochemical composition of Barre Grey granite is unique to the geological formation in Barre, Vermont.
The geochemical composition of granite is determined by the type of magma that formed it, as well as the geological processes that took place during its formation. These factors can vary widely depending on the location and geological history of the granite.
While there may be other types of granite in Europe and India that share similar physical characteristics, such as texture and color, they are likely to have distinct geochemical compositions.
In Europe, there are similar granites in color and texture, such as Sardinian Grey granite and Swedish Bohus grey granite.
In India, granite types like Black Galaxy granite and Chittoor Paradiso granite are similar to Barre Grey granite.
However, even within the same type of granite, color and texture can vary depending on the quarry and location of the stone.
The geochemical composition of granite, determined by the type of magma that formed it and geological processes, can vary widely depending on the location and geological history of the granite. Thus, it's important to evaluate each granite's unique geological history and composition, even if it has similar physical characteristics.
Black Galaxy granite is composed primarily of plagioclase feldspar, potassium feldspar, quartz, and biotite, giving it a distinct black color with small golden and silver flecks. It has an average density of 2.93 g/cm3 and a compressive strength of 190-200 MPa, making it suitable for milling wheat grains.
Chittoor Paradiso granite is composed of pinkish-red feldspar, quartz, and biotite, has an average density of 2.61 g/cm3 and a compressive strength of 160-190 MPa, and is also suitable for milling wheat grains.
Barre Grey granite is primarily composed of quartz, plagioclase feldspar, potassium feldspar, and biotite, has an average density of 2.65 g/cm3 and a compressive strength of 160-190 MPa, similar to Chittoor Paradiso granite.
However, its distinct gray color and fine-grained texture may affect the quality of flour produced from wheat grains. All three types of granite have low porosity, reducing the risk of flour contamination, but Black Galaxy and Chittoor Paradiso granite are suitable options for the Indian Subcontinent due to their availability, higher density and strength, ensuring a long lifespan of milling stones.
The mineral composition of the three granites:
Black Galaxy granite: The main minerals in Black Galaxy granite are plagioclase feldspar (oligoclase), biotite mica, and quartz. The rock may also contain small amounts of other minerals, such as hornblende and orthoclase feldspar.
Chittoor Paradiso granite: The main minerals in Chittoor Paradiso granite are orthoclase feldspar (microcline), plagioclase feldspar (oligoclase), biotite mica, and quartz. The rock may also contain small amounts of other minerals, such as garnet and zircon.
Barre Grey granite: The main minerals in Barre Grey granite are plagioclase feldspar (oligoclase), biotite mica, and quartz. The rock may also contain small amounts of other minerals, such as hornblende and orthoclase feldspar.
The general ranges of mineral composition for each of the three granites:
Black Galaxy granite: typically contains 40-60% quartz, 20-40% plagioclase feldspar (oligoclase), and 5-15% biotite mica.
Chittoor Paradiso granite: typically contains 30-50% quartz, 20-40% orthoclase feldspar (microcline), 20-40% plagioclase feldspar (oligoclase), and 5-15% biotite mica.
Barre Grey granite: typically contains 30-60% quartz, 20-40% plagioclase feldspar (oligoclase), and 5-15% biotite mica.
Please note that the mineral composition of a granite may vary depending on the specific location and origin of the rock.
Finally, some technical values for the three granites are given:
The approximate values for the coefficient of linear thermal expansion per degree Celsius (Cx10⁶) for the three granites:
Black Galaxy granite: 5.5 to 6.0
Chittoor Paradiso granite: 7.0 to 7.5
Barre Grey granite: 4.5 to 5.0
The approximate values for the compressive strength of the three granites according to ASTM C170 standard:
Black Galaxy granite: 190-200 MPa
Chittoor Paradiso granite: 160-190 MPa
Barre Grey granite: 160-190 MPa
The approximate values for the modulus of rupture according to ASTM C99 standard for the three granites:
Black Galaxy granite: 16-18 MPa
Chittoor Paradiso granite: 14-16 MPa
Barre Grey granite: 13-15 MPa
The approximate values for the flexural strength according to ASTM C880 standard for the three granites:
Black Galaxy granite: 15-17 MPa
Chittoor Paradiso granite: 12-14 MPa
Barre Grey granite: 12-14 MPa
Please note that these values may vary depending on the specific mineral composition and origin of the granite, but they provide a general idea of the flexural strength properties of each granite.
In conclusion, the time-honored craft of stone milling has prevailed for centuries as a preferred technique for creating premium flour. The process involves grinding grains between two stones, a method that effectively conserves the natural nutrients and aromatic qualities of the grains, ultimately producing flour that is both healthier and more flavorsome. Stones are deemed superior to other materials such as steel, as they emit less heat during the milling process, thereby ensuring that the grains remain unscathed, preserving their nutritional integrity.
While emery stones are the only option available at this point in time in India, other types of stones, such as granite, sandstone or basalt are also used globally for different grain types. When it comes to wheat grain stone milling, granite is definitely the most suitable option.
In the absence of alternatives, emery stones may suffice, although they may not be as efficient as other stones. In essence, stone milling endures as a preferred method for producing flour that is rich in nutrition, flavorful, and untainted by any form of artificial additives or chemicals.
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